Laser processing method for semiconductor wafer

ABSTRACT

A laser processing method for a semiconductor wafer including a groove forming step of applying a pulsed laser beam having an absorption wavelength to the semiconductor wafer along a division line formed on the semiconductor wafer to thereby form a laser processed groove along the division line on the semiconductor wafer, wherein the pulse width of the pulsed laser beam to be applied in the groove forming step is set to 2 ns or less, and the peak energy density per pulse of the pulsed laser beam is set less than or equal to an inflection point where the depth of the laser processed groove steeply increases with an increase in the peak energy density.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser processing method for asemiconductor wafer including a groove forming step of applying a laserbeam to the semiconductor wafer along a plurality of division lines tothereby form a plurality of laser processed grooves along these divisionlines on the semiconductor wafer.

2. Description of the Related Art

In a semiconductor device fabrication process, a plurality of crossingdivision lines called streets are formed on the front side of asubstantially disk-shaped semiconductor wafer such as a silicon waferand a gallium arsenide wafer to partition a plurality of regions wheredevices such as ICs and LSIs are respectively formed. The semiconductorwafer is divided into the individual devices along the division lines byusing a cutting apparatus or a laser processing apparatus, and thesedevices are widely used in various electrical equipment such as mobilephones and personal computers.

In general, a dicing apparatus is used as the cutting apparatus. Thedicing apparatus includes a cutting blade having a thickness of about 30to 300 pm. The cutting blade is obtained by bonding super abrasivegrains such as diamond and CBN (Cubic Baron Nitride) with metal orresin. Cutting is performed by rotating the cutting blade at a highspeed of about 30000 rpm and feeding the cutting blade into asemiconductor wafer.

On the other hand, the laser processing apparatus essentially includes achuck table for holding a semiconductor wafer, laser beam applying meansfor applying a pulsed laser beam to the semiconductor wafer held on thechuck table, and feeding means for relatively feeding the chuck tableand the laser beam applying means. The pulsed laser beam has anabsorption wavelength to the semiconductor wafer, and it is applied tothe semiconductor wafer along the division lines formed on the frontside of the semiconductor wafer to thereby form a plurality of laserprocessed grooves along these division lines. After forming the laserprocessed grooves, an external force is applied to the semiconductorwafer to break the semiconductor wafer along the laser processedgrooves, thereby dividing the semiconductor wafer into the individualdevices (see Japanese Patent Laid-open No. 2007-19252, for example).

SUMMARY OF THE INVENTION

In the case of cutting the semiconductor wafer by using the dicingapparatus having the cutting blade as mentioned above, each devicedivided from the semiconductor wafer has a die strength of 800 MPa. Tothe contrary, in the case of dividing the semiconductor wafer byperforming a conventional laser processing method, each device dividedfrom the semiconductor wafer has a die strength of 400 MPa. Such a lowdie strength causes a degradation in quality of electrical equipment.

It is therefore an object of the present invention to provide a laserprocessing method for a semiconductor wafer which can manufacture adevice having a high die strength.

In accordance with an aspect of the present invention, there is provideda laser processing method for a semiconductor wafer including a grooveforming step of applying a pulsed laser beam having an absorptionwavelength to the semiconductor wafer along a division line formed onthe semiconductor wafer to thereby form a laser processed groove alongthe division line on the semiconductor wafer, wherein the pulse width ofthe pulsed laser beam to be applied in the groove forming step is set to2 ns or less, and the peak energy density per pulse of the pulsed laserbeam is set less than or equal to an inflection point where the depth ofthe laser processed groove steeply increases with an increase in thepeak energy density.

According to the present invention, the pulse width of the pulsed laserbeam is set to 2 ns or less, and the peak energy density per pulse isset less than or equal to an inflection point where the depth of thelaser processed groove steeply increases with an increase in the peakenergy density. Thus, the die strength of each device can be improved to800 MPa or more.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a laser processing apparatusfor performing the laser processing method according to the presentinvention;

FIG. 2 is a perspective view of a semiconductor wafer supported throughan adhesive tape to an annular frame;

FIG. 3 is a block diagram of a laser beam applying unit;

FIG. 4 is a perspective view showing a groove forming step in the laserprocessing method according to the present invention;

FIG. 5A is a view for illustrating the groove forming step;

FIG. 5B is an enlarged sectional view of the semiconductor wafer at aposition where a laser processed groove is formed by the groove formingstep shown in FIG. 5A;

FIG. 6 is a schematic diagram for illustrating the amount of overlapbetween adjacent beam spots;

FIG. 7 is a perspective view of a dividing apparatus; and

FIGS. 8A and 8B are sectional side views for illustrating asemiconductor wafer dividing step.

FIG. 9 is a graph showing the relation between the peak energy densityand the depth of a laser processed groove in the case that a laser beamhaving a pulse width of 10 ps and a wavelength of 1064 nm is applied byone pulse to a silicon wafer;

FIG. 10 is a graph showing the relation between the peak energy densityand the depth of a laser processed groove in the case that a laser beamhaving a pulse width of 10 ps and a wavelength of 532 nm is applied byone pulse to a silicon wafer;

FIG. 11 is a graph showing the relation between the peak energy densityand the depth of a laser processed groove in the case that a laser beamhaving a pulse width of 10 ps and a wavelength of 355 nm is applied byone pulse to a silicon wafer; and

FIG. 12 is a graph showing the relation between the peak energy densityand the depth of a laser processed groove in the case that laser beamshaving a pulse width of 10 ps for each and different wavelengths of 1064nm and 532 nm are applied by one pulse for each to a GaAs wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described indetail with reference to the drawings. FIG. 1 is a schematic perspectiveview of a laser processing apparatus 2 for performing the laserprocessing method according to the present invention. The laserprocessing apparatus 2 includes a stationary base 4 and a first slideblock 6 supported to the stationary base 4 so as to be movable in an Xdirection in FIG. 1. The first slide block 6 is movable in a feedingdirection, i.e., in the X direction along a pair of guide rails 14 byfeeding means 12 including a ball screw 8 and a pulse motor 10.

A second slide block 16 is supported to the first slide block 6 so as tobe movable in a Y direction in FIG. 1. The second slide block 16 ismovable in an indexing direction, i.e., in the Y direction along a pairof guide rails 24 by indexing means 22 including a ball screw 18 and apulse motor 20. A chuck table 28 is supported through a cylindricalsupport member 26 to the second slide block 16. Accordingly, the chucktable 28 is movable both in the X direction and in the Y direction bythe feeding means 12 and the indexing means 22. The chuck table 28 isprovided with a pair of clamps 30 for clamping a semiconductor wafer(which will be hereinafter described) held on the chuck table 28 undersuction.

A column 32 is provided on the stationary base 4, and a casing 35 foraccommodating a laser beam applying unit 34 is mounted on the column 32.As shown in FIG. 3, the laser beam applying unit 34 includes a laseroscillator 62 such as a YAG laser oscillator or a YVO4 laser oscillator,repetition frequency setting means 64, pulse width adjusting means 66,and power adjusting means 68. A pulsed laser beam is oscillated by thelaser oscillator 62, and the power of the pulsed laser beam is adjustedby the power adjusting means 68. Focusing means 36 is mounted at thefront end of the casing 35 and includes a mirror 70 and a focusingobjective lens 72. The pulsed laser beam from the laser beam applyingunit 34 is reflected by the mirror 70 and next focused by the objectivelens 72 in the focusing means 36 so as to form a laser beam spot on thefront side (upper surface) of a semiconductor wafer W held on the chucktable 28.

Referring back to FIG. 1, imaging means 38 for detecting a processingarea of the semiconductor wafer W to be laser-processed is also providedat the front end of the casing 35 so as to be juxtaposed to the focusingmeans 36 in the X direction. The imaging means 38 includes an ordinaryimaging device such as a CCD for imaging the processing area of thesemiconductor wafer W by using visible light. The imaging means 38further includes infrared imaging means composed of infrared lightapplying means for applying infrared light to the semiconductor wafer W,an optical system for capturing the infrared light applied to thesemiconductor wafer W by the infrared light applying means, and aninfrared imaging device such as an infrared CCD for outputting anelectrical signal corresponding to the infrared light captured by theoptical system. An image signal output from the imaging means 38 istransmitted to a controller (control means) 40.

The controller 40 is configured by a computer, and it includes a centralprocessing unit (CPU) 42 for performing operational processing accordingto a control program, a read only memory (ROM) 44 for storing thecontrol program and so on, a random access memory (RAM) 46 for storingthe results of computation, etc., a counter 48, an input interface 50,and an output interface 52.

Reference numeral 56 denotes feed amount detecting means including alinear scale 54 provided along one of the guide rails 14 and a read head(not shown) provided on the first slide block 6. A detection signal fromthe feed amount detecting means 56 is input into the input interface 50of the controller 40. Reference numeral 60 denotes index amountdetecting means including a linear scale 58 provided along one of theguide rails 24 and a read head (not shown) provided on the second slideblock 16. A detection signal from the index amount detecting means 60 isinput into the input interface 50 of the controller 40. An image signalfrom the imaging means 38 is also input into the input interface 50 ofthe controller 40. On the other hand, control signals are output fromthe output interface 52 of the controller 40 to the pulse motor 10, thepulse motor 20, and the laser beam applying unit 34.

As shown in FIG. 2, the front side of the semiconductor wafer W as aworkpiece to be processed by the laser processing apparatus 2 is formedwith a plurality of first streets S1 and a plurality of second streetsS2 perpendicular to the first streets S1, thereby partitioning aplurality of rectangular regions where a plurality of devices D arerespectively formed. The semiconductor wafer W is attached to a dicingtape T as an adhesive tape, and the dicing tape T is supported at itsouter circumferential portion to an annular frame F. Accordingly, thesemiconductor wafer W is supported through the dicing tape T to theannular frame F. The semiconductor wafer W is held on the chuck table 28shown in FIG. 1 under suction in the condition where the annular frame Fis clamped by the clamps 30.

A laser processing method for the semiconductor wafer W will now bedescribed with reference to FIGS. 4 to 12. As shown in FIGS. 4 and 5A, apulsed laser beam 37 having an absorption wavelength to thesemiconductor wafer W is focused by the focusing means 36 onto the frontside of the semiconductor wafer W. At the same time, the chuck table 28is moved in a direction shown by an arrow X1 in FIG. 5A at apredetermined feed speed from one end (left end as viewed in FIG. 5A) ofa predetermined one of the first streets S1.

When the other end (right end as viewed in FIG. 5A) of thispredetermined first street S1 reaches the laser beam applying positionof the focusing means 36, the application of the pulsed laser beam isstopped and the movement of the chuck table 28 is also stopped. As aresult, a laser processed groove 74 is formed on the front side of thesemiconductor wafer W along this predetermined first street S1 as shownin FIG. 5B. Such a groove forming step of forming the laser processedgroove 74 is performed similarly along the other first streets S1. Afterthus performing the groove forming step along all of the first streetsS1, the chuck table 28 is rotated 90° to similarly perform the grooveforming step along all of the second streets S2 perpendicular to thefirst streets S1. As a result, the laser processed grooves 74 are formedalong all of the first and second streets S1 and S2 on the front side ofthe semiconductor wafer W.

In the laser processing method according to the present invention, asshown in FIG. 6, the overlap amount OL of the adjacent spots S of thepulsed laser beam in the feeding direction is preferably adjusted tofall within the range of 16D1/20≦OL≦19D1/20, where D1 is the spotdiameter of the pulsed laser beam, by optimally setting the repetitionfrequency, pulse width, spot diameter D1, and feed speed of the pulsedlaser beam applied from the focusing means 36.

After forming the laser processed grooves 74 along all of the first andsecond streets S1 and S2 as mentioned above, a wafer dividing step isperformed by using a dividing apparatus 80 shown in FIG. 7 in such amanner that the semiconductor wafer W is divided into the individualdevices (chips) D along all of the laser processed grooves 74. Thedividing apparatus 80 shown in FIG. 7 includes frame holding means 82for holding the annular frame F and tape expanding means 84 forexpanding the dicing tape T supported to the annular frame F held by theframe holding means 82.

The frame holding means 82 includes an annular frame holding member 86and a plurality of clamps 88 as fixing means provided on the outercircumference of the frame holding member 86. The upper surface of theframe holding member 86 functions as a mounting surface 86 a formounting the annular frame F thereon. The annular frame F mounted on themounting surface 86 a is fixed to the frame holding member 86 by theclamps 88. The frame holding means 82 is supported by the tape expandingmeans 84 so as to be vertically movable.

The tape expanding means 84 includes an expanding drum 90 providedinside of the annular frame holding member 86. The expanding drum 90 hasan outer diameter smaller than the inner diameter of the annular frame Fand an inner diameter larger than the outer diameter of thesemiconductor wafer W attached to the dicing tape T supported to theannular frame F. The expanding drum 90 has a supporting flange 92integrally formed at the lower end of the drum 90. The tape expandingmeans 84 further includes driving means 94 for vertically moving theannular frame holding member 86. The driving means 94 is composed of aplurality of air cylinders 96 provided on the supporting flange 92. Eachair cylinder 96 is provided with a piston rod 98 connected to the lowersurface of the frame holding member 86. The driving means 94 composed ofthe plural air cylinders 96 functions to vertically move the annularframe holding member 86 so as to selectively take a reference positionwhere the mounting surface 86 a is substantially equal in height to theupper end of the expanding drum 90 and an expansion position where themounting surface 86 a is lower in height than the upper end of theexpanding drum 90 by a predetermined amount.

The wafer dividing step using the dividing apparatus 80 will now bedescribed with reference to FIGS. 8A and 8B. As shown in FIG. 8A, theannular frame F supporting the semiconductor wafer W through the dicingtape T is mounted on the mounting surface 86 a of the frame holdingmember 86 and fixed to the frame holding member 86 by the clamps 88. Atthis time, the frame holding member 86 is set at the reference positionwhere the height of the mounting surface 86 a is substantially the sameas that of the upper end of the expanding drum 90.

Thereafter, the air cylinders 96 are driven to lower the frame holdingmember 86 to the expansion position shown in FIG. 8B. Accordingly, theannular frame F fixed to the mounting surface 86 a of the frame holdingmember 86 is also lowered, so that the dicing tape T supported to theannular frame F comes into abutment against the upper end of theexpanding drum 90 and is expanded mainly in the radial direction of theexpanding drum 90 as shown in FIG. 8B. As a result, a tensile force isradially applied to the semiconductor wafer W attached to the dicingtape T. When a tensile force is radially applied to the semiconductorwafer W, the semiconductor wafer W is broken along the laser processedgrooves 74, thereby dividing the semiconductor wafer W into theindividual semiconductor chips (devices) D.

The die strength of each device formed by a conventional laserprocessing method is as low as 400 MPa, causing a degradation in qualityof electrical equipment. To cope with this problem, attention has beenpaid to a peak energy density per pulse in laser processing, and therelation between the peak energy density and the depth of a laserprocessed groove has been examined.

FIG. 9 shows the relation between the peak energy density and the depthof a laser processed groove in the case that a laser beam having a pulsewidth of 10 ps and a wavelength of 1064 nm is applied by one pulse to asilicon wafer. In FIG. 9, the horizontal axis represents the peak energydensity (GW/cm²) graduated logarithmically, and the vertical axisrepresents the depth (nm) of the laser processed groove. As apparentfrom FIG. 9, when the peak energy density is small, the depth of thelaser processed groove gradually increases with an increase in the peakenergy density, whereas when the peak energy density becomes a point Aor more, the depth of the laser processed groove steeply increases withan increase in the peak energy density. In the present invention, thispoint A is defined as an inflection point. The peak energy density atthe point A is about 200 GW/cm². According to the present invention, ithas been found that the die strength of each device can be improved to800 MPa or more by setting the pulse width to a predetermined value orless and also setting the peak energy density per pulse to theinflection point A or less to thereby control the depth of the laserprocessed groove per pulse.

FIG. 10 shows the relation between the peak energy density and the depthof a laser processed groove in the case that a laser beam having a pulsewidth of 10 ps and a wavelength of 532 nm is applied by one pulse to asilicon wafer. As in the case of using the laser beam having awavelength of 1064 nm shown in FIG. 9, there is an inflection point B inthe case of using the laser beam having a wavelength of 532 nm as shownin FIG. 10. By applying this laser beam to the silicon wafer with thepeak energy density less than or equal to the inflection point B, thedie strength of each device can be improved to 800 MPa or more. Theinflection point B is about 200 GW/cm².

FIG. 11 shows the relation between the peak energy density and the depthof a laser processed groove in the case that a laser beam having a pulsewidth of 10 ps and a wavelength of 355 nm is applied by one pulse to asilicon wafer. As in the cases of using the laser beam having awavelength of 1064 nm shown in FIG. 9 and the laser beam having awavelength of 532 nm shown in FIG. 10, there is an inflection point C inthe case of using the laser beam having a wavelength of 355 nm as shownin FIG. 11. By applying this laser beam to the silicon wafer with thepeak energy density less than or equal to the inflection point C, thedie strength of each device can be improved to 800 MPa or more. Theinflection point C is about 200 GW/cm².

FIG. 12 shows the relation between the peak energy density and the depthof a laser processed groove in the case that laser beams having a pulsewidth of 10 ps for each and different wavelengths are applied by onepulse for each to a GaAs wafer. In FIG. 12, reference numeral 100denotes the laser beam having a wavelength of 1064 nm, and referencenumeral 102 denotes the laser beam having a wavelength of 532 nm. Asapparent from FIG. 12, in the case of using the GaAs wafer, there are aremarkable inflection point D (wavelength: 1064 nm) and a remarkableinflection point E (wavelength: 532 nm) where the depth of the laserprocessed groove steeply increases. Thus, also in the case of using theGaAs wafer, the die strength of each device can be improved by settingthe peak energy density per pulse to the inflection point or less. InFIG. 12, the inflection point D is about 800 GW/cm², and the inflectionpoint E is about 70 GW/cm².

The following test was carried out to confirm the above findings by thepresent invention, determine the preferable pulse width of the pulsedlaser beam, and examine the processing conditions for attaining a diestrength of 800 MPa or more.

Pulsed laser beams having wavelengths of 1064 nm, 532 nm, and 355 nmwere used and the pulse width of each pulsed laser beam was changed to30 ns, 10 ns, 5 ns, 3 ns, 2 ns, 1 ns, 100 ps, 50 ps, and 10 ps. In eachpulse width, the power was changed to experimentally obtain an energyper pulse for attaining desired laser processing. This energy wasdivided by the pulse width and the spot area to thereby calculate a peakenergy density. Then, the relation between the pulse width, the peakenergy density, and the die strength was examined.

The peak energy density is given by the following equation.Peak energy density (W/cm²)=Average power (W)/(Repetition frequency(Hz)×Spot area (cm²)×Pulse width (s))

As a result, the following results were similarly obtained for all ofthe pulsed laser beams having the wavelengths of 1064 nm, 532 nm, and355 nm.

Test 1

The semiconductor wafer was processed to form the laser processedgrooves under the following conditions.

Repetition frequency: 10 kHz

Average power: 0.1 W

Pulse width: 2 ns

Spot diameter: 10 μm

Feed speed: 10 mm/s

Peak energy density: 6.35 GW/cm²

The semiconductor wafer was next divided along the laser processedgrooves to obtain the individual devices. The die strength of eachdevice was measured to attain 800 MPa.

Test 2

The semiconductor wafer was processed to form the laser processedgrooves under the following conditions.

Repetition frequency: 100 kHz

Average power: 0.1 W

Pulse width: 10 ps

Spot diameter: 10 μm

Feed speed: 100 mm/s

Peak energy density: 63.66 GW/cm²

The semiconductor wafer was next divided along the laser processedgrooves to obtain the individual devices. The die strength of eachdevice was measured to attain 1800 MPa.

Test 3

The semiconductor wafer was processed to form the laser processedgrooves under the following conditions.

Repetition frequency: 100 kHz

Average power: 0.3 W

Pulse width: 10 ps

Spot diameter: 10 μm

Feed speed: 100 mm/s

Peak energy density: 190.9 GW/cm²

The semiconductor wafer was next divided along the laser processedgrooves to obtain the individual devices. The die strength of eachdevice was measured to attain 1000 MPa.

Test 4

The semiconductor wafer was processed to form the laser processedgrooves under the following conditions.

Repetition frequency: 100 kHz

Average power: 0.4 W

Pulse width: 10 ps

Spot diameter: 10 μm

Feed speed: 100 mm/s

Peak energy density: 254.6 GW/cm²

The semiconductor wafer was next divided along the laser processedgrooves to obtain the individual devices. The die strength of eachdevice was measured to attain 500 MPa.

Test 5

The semiconductor wafer was processed to form the laser processedgrooves under the following conditions.

Repetition frequency: 10 kHz

Average power: 0.2 W

Pulse width: 3 ns

Spot diameter: 10 μm

Feed speed: 10 mm/s

Peak energy density: 8.2 GW/cm²

The semiconductor wafer was next divided along the laser processedgrooves to obtain the individual devices. The die strength of eachdevice was measured to attain 500 MPa.

As apparent from the results of Tests 1 to 3, it was confirmed that adie strength of 800 MPa or more can be obtained in the case that thepulse width is set to 2 ns or less and the peak energy density is set to200 GW/cm² or less. This value of 200 GW/cm² for the peak energy densitysubstantially corresponds to each of the inflection points A, B, and Crespectively shown in FIGS. 9, 10, and 11.

However, as apparent from the result of Test 5, although the peak energydensity is less than 200 GW/cm², the die strength of each device is 500MPa in the case that the pulse width is set to 3 ns. That is, when thepulse width is greater than 2 ns, it was confirmed that the improvementin the die strength of each device is insufficient. It is thereforenecessary to set the pulse width to 2 ns or less. Further, as apparentfrom the result of Test 4, the peak energy density is greater than eachof the inflection points A, B, and C. In this case, although the pulsewidth is less than 2 ns, it was confirmed that the improvement in thedie strength of each device is insufficient.

Further as described above with reference to FIG. 6, the overlap rate ofthe adjacent spots S in the feeding direction is preferably set in therange of 16/20 to 19/20 with respect to spot diameter. Further, the spotdiameter of the laser beam focused on the semiconductor wafer W ispreferably set in the range of 5 to 15 μm.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. A laser processing method for a semiconductorwafer comprising a groove forming step of applying a pulsed laser beamhaving an absorption wavelength to said semiconductor wafer along adivision line formed on said semiconductor wafer to thereby form a laserprocessed groove along said division line on said semiconductor wafer,wherein the pulse width of said pulsed laser beam to be applied in saidgroove forming step is set to 2 ns or less, and the peak energy densityper pulse of said pulsed laser beam is set less than or equal to aninflection point where the depth of the laser processed groove steeplyincreases with an increase in said peak energy density, wherein thelaser processed groove is formed on an upper surface of thesemiconductor wafer and extends into the wafer.
 2. The laser processingmethod of claim 1, wherein said pulsed laser beam is applied to thesemiconductor wafer as a series of overlapping spots having a diameter D, and wherein an overlap amount OL of adjacent spots is set within arange of 0.8D≦OL≦0.95D.
 3. The laser processing method of claim 1,wherein said peak energy density is within a range of about 6.35 GW/cm²to about 200 GW/cm².
 4. The laser processing method of claim 3, whereinsaid peak energy density is approximately 200 GW/cm².
 5. The laserprocessing method of claim 1, wherein said pulsed laser beam has awavelength of 532 nm.
 6. The laser processing method of claim 1, whereinsaid pulsed laser beam has a wavelength of 355 nm.